A major source of atmospheric air pollution is the exhaust gas from automobile engines. A present approach to control this general problem is to modify engine operation parameters through spark timing control systems to alter combustion characteristics of the internal combustion engine, thereby reducing exhaust emissions at the disadvantage of loss of economy and performance.
Most prior art vacuum spark advance control systems have some sort of a vacuum servo controlling the advance or retard setting of the engine distributor as a function of carburetor spark port vacuum to provide good engine performance as well as fuel economy during the difference operating conditions of the engine. These vacuum servos, in their simplest form, generally consist of a housing divided into atmospheric pressure and vacuum chambers by a flexible diaphragm connected to the distributor breaker plate. The diaphragm and breaker plate are normally spring biased to the lowest advance or retard spark timing setting, and carburetor spark port vacuum normally urges the diaphragm in a spark timing advance direction upon opening of the carburetor throttle valve corresponding to increasing engine speed.
With the above construction, during rapid acceleration, the drop-in vacuum at the carburetor spark port permits atmospheric pressure acting in the opposing chamber of the distributor's servo to immediately move the distributor breaker plate to a lower advance setting (retarding the spark), that is, a setting that is best to meet engine performance requirements. On the other hand, however, upon return to normal operation and gradual reacceleration or deceleration of the engine, an increase in vacuum at the carburetor spark port causes an immediate return movement of the vacuum servo diaphragm thereby causing a higher engine spark timing advance. This provides a longer burning time for the fuel mixture before the optimum top or near top dead center position of the piston is attained, generally providing the most desirable economic operation. However, this longer time permits the build-up of higher combustion temperatures and pressures, which are undesirable insofar as the production of oxides of nitrogen and other undesirable elements of exhaust emissions are concerned. It can be seen, therefore, that the conventional spark timing control system generally provides good performance and fuel economy, but does not necessarily minimize the output of undesirable exhaust gas emissions.
Other systems are known such as the type shown in U.S. Pat. No. 3,606,871 which created an improvement over the aforementioned devices. The above-mentioned patent shows a vacuum regulated mechanical device which includes a one-way check valve and an orifice in parallel flow circuits connected between the carburetor spark port and the vacuum servo mechanism. During rapid vehicle accelerations, the check valve unseats to provide a quick equalization of the pressure at the servo to the spark portion vacuum thereby lowering the spark advance setting to avoid detonation. Detonation is pre-ignition spark knock or ping and is a result of spontaneous ignition of the explosive gasoline-air mixture which under certain circumstances occurs in the cylinders of the internal combustion engine. Detonation reduces power output, causes overheating, unduly stresses the cylinder head and pistons, and is generally objectionable from the noise and vibration standpoint. Upon a momentary deceleration condition of operation, with the subsequent return toward former operating conditions, the orifice provides a slow build-up of the vacuum level at the servo to equal that at the spark port so that the advance setting only slowly returns to normal. This results in lower peak combustion temperatures and pressures and a lower emission level of engine pollutants. However, the above-referenced system is poor for fuel economy. The slower spark advance build-up due to the orifice bleed of vacuum causes late combustion of air-fuel mixture and this combustion is generally at a point past optimum efficiency, i.e., into the expansion cycle of the engine.
An even later patent, U.S. Pat. No. 3,698,366, overcame the disadvantageous function of the device described in U.S. Pat. No. 3,606,871 by providing a rapid return of the spark timing advance setting to essentially the former level, after a momentary deceleration, to improve the fuel economy.
The prior art described above utilizing vacuum as a control means has the additional disadvantage of suffering from a degraded performance as a result of changes in altitude as well as high vehicle speed. Commonly assigned U.S. Pat. application Ser. No. 329,289 entitled "Nonlinear Vacuum Spark Advance System", filed Dec. 2, 1973, provides a partially altitude compensated vacuum control for the distributor vacuum spark advance system. Compensation is accomplished by using a predetermined value of spark advance at moderate and low speeds using the spark port vacuum as a first signal source. At higher speeds this system provides a switching function to a secondary signal source of vacuum, namely, the EGR signal. The second signal source vacuum is then utilized to control or regulate the distributor servo vacuum spark advance. This secondary vacuum signal source utilized to regulate the distributor at higher speeds does not offer altitude compensation. Therefore, at high vehicle speeds and at increased altitudes, this valve does not have the capability of maintaining a full vacuum advance due to the degradation of the EGR vacuum signal, as altitude changes.
The approach discussed in the prior art devices in providing a spark advance vacuum signal to the distributor has resulted in significant reduction in fuel economy as well as a significant drop in the level of performance of the internal combustion engine. All automobile internal combustion engines suffer degraded performance when operated at higher speeds and at higher altitudes due to the continuous reduction in the spark port vacuum signal which was heretofore provided directed to the vacuum spark advance diaphragm mechanism. Some altitude compensation has been provided at low speeds by limiting the spark advance at a predetermined level at low and moderate speeds and then switching to a non-altitude compensated vacuum signal, namely, the EGR signal, thereby effectively providing a limited amount of altitude compensation at low and moderate speeds. At higher speeds, however, none of the prior art devices offer a regulated altitude compensated signal to provide an altitude compensated spark advance vacuum signal to the distributor vacuum servo.